The “last mile” provides consumers a direct connection with a telecom carrier. The standard telecom network configuration in the “last mile” is called a star or hub configuration. Households typically have two or more copper pairs that converge into a single hub from which either higher cable count twisted pair cable, higher speed cable (such as T1/T3 in North America or E1/E3 around the world) or fibre transports the signals back to a CO (central office). The remote sites are often powered cabinets in residential districts.
Typically the largest single asset that telecom carriers possess is the twisted pairs that go into each household in their area. To replace this asset is enormously costly and has been delayed in the hopes that a much less expensive access technology would present itself. Wireless solutions are currently available but serious questions regarding security and available bandwidth persist. Furthermore, bypassing the “last mile” with a wireless solution renders all investment in the already installed twisted pairs useless. Most of the investment in the twisted pairs has been amortized since initial installation but telecom carriers like to maximize their Return On Investment (ROI).
Many telecom carriers offer some form of DSL (Digital Subscriber Line) service that enables Internet access over copper twisted pair phone lines. There are many versions of DSL with various levels of transmission bandwidth over various distances. Examples of DSL technologies include ADSL (Asynchronous DSL), SDSL (Symmetric DSL), and VDSL (Very high bit rate DSL). The latest is VDSL2 (Very high bit rate DSL version 2), which enables symmetric DSL service at about 50 Mb/s (mega bits per second) rates over short distances (1-2 kft or about ½ km) as shown in FIG. 7. Generally, as the bandwidth increases, the distance over which that bandwidth can be transmitted decreases. There are also other technologies that use Ethernet over twisted pair. Technology such as VoIP (Voice over Internet Protocol) has enabled competition from cable providers who have their own networks, including their own ‘last mile’, and are well funded via their entertainment offerings. There is a need for telecom carriers to provide much higher bandwidth at much greater distances from the CO than they currently do.
Fibre-based solutions are not practical solutions to the problems discussed herein. Fiber-to-the-Node (FTTN) or Fiber-to-the-Curb (FTTC) architectures can move DSLAMs (DSL Access Multiplexers) closer to the subscribers, thereby increasing the bandwidth available. However, the fiber, cabinet for the equipment, real estate for the cabinet, and power do not already exist in the telecom network. Installing this infrastructure is an enormous logistical effort and is incredibly expensive. Fiber-to-the-Premises (FTTP) takes the FTTC/N approach one step further by taking new fiber directly to the customer's house. The fiber does not already exist in the network so it has to be installed, again at great expense. The technologies that are being used for these rollouts are generally based on PON (Passive Optical Network) architectures. These architectures have been around for many years and have seen very little adoption until VoIP telephony became commercially viable.
Recently an industry initiative took another look at a method of sharing bandwidth, specifically on twisted pair networks. The premise is that if an original high bandwidth signal is split into several pieces and sent over several pairs as a single transmission path, then the bandwidth can be increased significantly at virtually every distance. This method assumes that there are additional pairs available for this purpose. Protocol-based overhead is inserted on each physical wire so that the signal, which is transmitted in several pieces, can be put back together in the correct order at the far end. This process is called ‘bonding’ and is being specified under the moniker G.BOND (ITU specifications G.998.1—ATM, 2—Ethernet, 3—inverse multiplexing). The difficulty in applying this method to the existing cable plant is that there are generally between 2 and 4 pairs going into each residence. If the current ADSL capability of 4 Mb/s at an average distance of 2.5 km from the CO is used, then this yields a maximum of 16 Mb/s available to each house. As it is more common that there are only 2 pairs in residences the maximum bandwidth would be 8 Mb/s in the same scenario. This is still considered to be very tight for video transmission, even with MPEG4 compression (which is currently not very common) as the bandwidth, jitter profile and latency, needs to be guaranteed.
Modifications to the twisted pair cable plant (i.e.: installing additional twisted pairs throughout the network) may be just as expensive as replacing it with fibre.